Make WP Captions Responsive to image widths.

wp-caption-responsive.js

function caption_responsive(){
	//// Make captions responsive 
	// we want to change the width of the figcaptions within figures 
	$('.wp-caption').each(function(){
		var caption = $(this),
			figcaption = caption.find('figcaption').first(),
			img = caption.find('img[class*="wp-image-"]').first();

		if (img.length) {
	        // wait for the image to load or it might have no dimensions
	        img.load(function() {
	        	// Clear the default WP Core styling 
	        	caption.removeAttr('style');
	            figcaption.css("width", img.width())
	        });
	    }
	});
}

jQuery(document).ready(function($) {
  caption_responsive();

	$(window).resize(function() {
		caption_responsive();
	});
  
});